How to make a SQL "IF-THEN-ELSE" statement - sql-server

Question

I've seen other questions about SQL If-then-else stuff, but I'm not seeing how to relate it to what I'm trying to do. I've been using SQL for about a year now but only basic stuff and never this.
If I have a SQL table that looks like this
| Name | Version | Category | Value | Number |
|:-----:|:-------:|:--------:|:-----:|:------:|
| File1 | 1.0 | Time | 123 | 1 |
| File1 | 1.0 | Size | 456 | 1 |
| File1 | 1.0 | Final | 789 | 1 |
| File2 | 1.0 | Time | 312 | 1 |
| File2 | 1.0 | Size | 645 | 1 |
| File2 | 1.0 | Final | 978 | 1 |
| File3 | 1.0 | Time | 741 | 1 |
| File3 | 1.0 | Size | 852 | 1 |
| File3 | 1.0 | Final | 963 | 1 |
| File1 | 1.1 | Time | 369 | 2 |
| File1 | 1.1 | Size | 258 | 2 |
| File1 | 1.1 | Final | 147 | 2 |
| File2 | 1.1 | Time | 741 | 2 |
| File2 | 1.1 | Size | 734 | 2 |
| File2 | 1.1 | Final | 942 | 2 |
| File3 | 1.1 | Time | 997 | 2 |
| File3 | 1.1 | Size | 997 | 2 |
| File3 | 1.1 | Final | 985 | 2 |
How can I write a SQL IF, ELSE statement that creates a new column called "Replication" that follows this rule:
A = B + 1 when x = 1
else
A = B
where A = the number we will use for the next Number
B = Max(Number)
x = Replication count (this is the number of times that a loop is executed. x=i)
The results table will look like this:
| Name | Version | Category | Value | Number | Replication |
|:-----:|:-------:|:--------:|:-----:|:------:|:-----------:|
| File1 | 1.0 | Time | 123 | 1 | 1 |
| File1 | 1.0 | Size | 456 | 1 | 1 |
| File1 | 1.0 | Final | 789 | 1 | 1 |
| File2 | 1.0 | Time | 312 | 1 | 1 |
| File2 | 1.0 | Size | 645 | 1 | 1 |
| File2 | 1.0 | Final | 978 | 1 | 1 |
| File1 | 1.0 | Time | 369 | 1 | 2 |
| File1 | 1.0 | Size | 258 | 1 | 2 |
| File1 | 1.0 | Final | 147 | 1 | 2 |
| File2 | 1.0 | Time | 741 | 1 | 2 |
| File2 | 1.0 | Size | 734 | 1 | 2 |
| File2 | 1.0 | Final | 942 | 1 | 2 |
| File1 | 1.1 | Time | 997 | 2 | 1 |
| File1 | 1.1 | Size | 997 | 2 | 1 |
| File1 | 1.1 | Final | 985 | 2 | 1 |
| File2 | 1.1 | Time | 438 | 2 | 1 |
| File2 | 1.1 | Size | 735 | 2 | 1 |
| File2 | 1.1 | Final | 768 | 2 | 1 |
| File1 | 1.1 | Time | 786 | 2 | 2 |
| File1 | 1.1 | Size | 486 | 2 | 2 |
| File1 | 1.1 | Final | 135 | 2 | 2 |
| File2 | 1.1 | Time | 379 | 2 | 2 |
| File2 | 1.1 | Size | 943 | 2 | 2 |
| File2 | 1.1 | Final | 735 | 2 | 2 |
EDIT: Based on the answer by Sean Lange, this is my 2nd attempt at a solution:
SELECT COALESCE(MAX)(Number) + CASE WHEN Replication = 1 then 1 else 0, 1) FROM Table
The COALESCE is in there for when there is no value yet in the Number column.

Answer 1

The IF/Else construct is used to control flow of statements in t-sql. You want a case expression, which is used to conditionally return values in a column.
https://msdn.microsoft.com/en-us/library/ms181765.aspx
Yours would be something like:
case when x = 1 then A else B end as A

Answer 2

As SeanLange pointed out in this case it would be better to use an CASE/WHEN but to illustrate how to use If\ELSE the way to do it in sql is like this:
if x = 1
BEGIN
---Do something
END
ELSE
BEGIN
--Do something else
END
I would say the best way to know the difference and when to use which is if you are writing a query and want a different field to appear based on a certain condition, use case/when. If a certain condition will cause a series of steps to happen then use if/else